Detention dam

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A detention dam is a dam built to catch surface runoff and stream water flow to regulate the water flow in areas below the dam. [1] Detention dams are commonly used to reduce the damage caused by flooding or to manage the flow rate through a channel. [2] Detention dams can also be constructed to replenish groundwater and trap sediment. [2] Detention dams are one of three classifications of dams: storage dams, diversion dams, and detention dams. [2] Storage dams store water for extended times for irrigation, livestock, municipal water supply, recreation, and hydroelectric power generation. Diversion dams raise the water level to redirect the water to a designated location. The diverted water typically supplies irrigation systems or reservoirs. [2]

Contents

Purposes

People build detention dams for purposes that include: flood control, flow rate restriction, subsurface water replenishment, and sediment trapping. [2] Detention dams are common in flood prone areas for flood control. A detention dam is built at an elevation above the flood prone zone. Flood water collects in the basin above the dam and is released at a flow rate the flood zone and channel can accommodate. Channels can include dikes, canals, streams, drain pipes, and rivers. The basin above a flood detention dam should remain at the lowest water level to prevent overtopping. [2] Overtopping is when the water level behind the dam exceeds the dam crest height. [3] The dam crest is the top edge of the dam. Overtopping is caused by extreme flooding or severe waves. The severe waves can be a result of high winds, landslides, and earthquakes. [3] The detention dam design must take into account the probability of overtopping occurring and be designed accordingly. Detention dams built to restrict flow rate regulate the amount water released into channels. [2] Detentions dams used to replenishment the subsurface water or groundwater hold surface runoff to allow the ground to absorb the water. [2] Groundwater is water that has drained into an aquifer due to the force of gravity. An aquifer is layers of permeable soils and rocks below the earth's surface that allow water to accumulated between the rocks and soils. [4] Debris dams are a type of detention dam used to collect sediment to prevent it from flowing into areas where large sediment buildup may be damaging. [2]

Design

Detention dams have two basic designs. [5] The detention dam can be made from concrete or masonry, usually with a metal reinforcing substructure. The concrete or masonry style dam commonly has a cross sectional shape similar to a right triangle with the sloping face pointing downstream and the perpendicular face pointing upstream. Detention dams can also be made from rock or earth to form a gravity embankment style dam. The cross sectional shape of an earth and rock gravity embankment style dam closely resembles an equal lateral triangle with the angled sides facing upstream and downstream. [5] Modern detentions dam designs incorporate safety factors that account for and compensate for the probability of failure. [3]

Flood Detention Dam Design

Flood detention dams are commonly used as part of a flood or storm water detention system. [6] Flood detention systems combine detention basins, detention dams, and channels to efficiently collect and regulate runoff. Detention facilities not only regulate the amount of released water, but control release water quality. Systems monitor runoff to prevent contaminates and debris from damaging lakes, rivers, and wetlands. Flood detention dams are constructed methodically. Engineers analyze watershed topographical data, hydrological records, and geological structure for the area to determine the most effective locations flood detention dams. The analyzed watershed topographical data, hydrological records, and geological structure display the potential storage capacity, environmental impacts, and physical limitations of the area. This produces models that simulate the effectiveness of possible flood detention dam locations and designs. The general design for a flood detention dam has a cross sectional shape of a trapezoid, where the longer of the parallel sides is the base of the dam, and the angled sides face upstream and downstream. The flood detention dam has an opening at the top to release the flood water at a controlled rate that the channels below can accommodate. Flood detention dam models help determine the necessary dam height and overflow opening size to prevent over-topping. [6]

Disadvantages

Detention dams can cause injury and damage if they are not built and maintained correctly. Poorly maintained and older detention dams can pose a reliability threat because they may not meet the current structural safety and hydraulic requirements. [3] For example, a detention dam in a populated area that does not conform to the current structural safety and hydraulic requirements has a high probability of failing. [7] If severe flood were to occur, the nonconforming detention dam could be overtopped and breached resulting in injury and damage of the surrounding populated area below the detention dam. [7]

Examples

Sediment detention dams

The area of Fifteenmile Creek, Wyoming was flagged by the United States Department of the Interior Bureau of Land Management in the 1960s because the area was in need of an aggressive sediment control system. [8] Over the course of 10 years, US$2 million was spent to construct 34 sediment detention dams, 110 reservoirs, and 21 spreader dikes to manage the sediment issue. The sediment control system was intended to reduce the amount of suspended sediment in the Bighorn River. The high sediment concentration in the Bighorn River was largely attributed to the drainage from the Fifteenmile Creek erosion. The control system was intended to reduce the sediment amount by 25%. However, 20 years after installation the control system was analyzed and uncovered that the detention dams had been improperly maintained, resulting in sediment detention failure. In addition to poor maintenance, the functional lives of the detention dams were shortened because of the location and climate. As a result, people who live downstream of the Bighorn River have to cope with the effects of the sediment. The high amounts of sediment released from the failed detention dams and control system have increased the cost to filter municipal water due to suspended sediment in the Bighorn River. The high sediment deposits have also damaged fisheries and reduced the amount of water that can be stored in a downstream reservoir. [8]

Flood detention dams

In San Antonio, Texas, the Olmos Creek detention dam was constructed as a flood detention dam. [9] Even though the Olmos Creek detention dam’s primary purpose was as a flood detention dam, the dam also acts as debris or sediment detention dam to trap pollutants from entering regional the water supply. The Olmos Creek detention dam is unique because it is located in an urban area with a large floodplain and the area around the detention dam is used as a recreational and wildlife area. As a result, the Olmos Creek detention dam is a multipurpose facility that can handle large floods, trap pollutants, and provide a recreational and wildlife area for the community. [9]

Related Research Articles

Flood Natural disaster caused by water overflow

A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrology and are of significant concern in agriculture, civil engineering and public health. Human changes to the environment often increase the intensity and frequency of flooding, for example land use changes such as deforestation and removal of wetlands, changes in waterway course or flood controls such as with levees, and larger environmental issues such as climate change and sea level rise. In particular climate change's increased rainfall and extreme weather events increases the severity of other causes for flooding, resulting in more intense floods and increased flood risk.

Diversion dam

A diversion dam is a dam that diverts all or a portion of the flow of a river from its natural course. Diversion dams do not generally impound water in a reservoir; instead, the water is diverted into an artificial water course or canal, which may be used for irrigation or return to the river after passing through hydroelectric generators, flow into a different river or be itself dammed forming an onground or groundwater reservoir or a storm drain.

Storm drain Infrastructure for draining excess rain and ground water from impervious surfaces such as paved streets

A storm drain, storm sewer, surface water drain/sewer, or stormwater drain is infrastructure designed to drain excess rain and ground water from impervious surfaces such as paved streets, car parks, parking lots, footpaths, sidewalks, and roofs. Storm drains vary in design from small residential dry wells to large municipal systems.

Santa Ana River River in California, United States

The Santa Ana River is the largest river entirely within Southern California in the United States. It rises in the San Bernardino Mountains and flows for most of its length through San Bernardino and Riverside Counties, before cutting through the northern Santa Ana Mountains via Santa Ana Canyon and flowing southwest through urban Orange County to drain into the Pacific Ocean. The Santa Ana River is 96 miles (154 km) long, and its drainage basin is 2,650 square miles (6,900 km2) in size.

In hydrology, discharge is the volumetric flow rate of water that is transported through a given cross-sectional area. It includes any suspended solids, dissolved chemicals, or biologic material in addition to the water itself. Terms may vary between disciplines. For example, a fluvial hydrologist studying natural river systems may define discharge as streamflow, whereas an engineer operating a reservoir system may equate it with outflow, contrasted with inflow.

Spillway Structure for controlled release of flows from a dam or levee

A spillway is a structure used to provide the controlled release of water from a dam or levee downstream, typically into the riverbed of the dammed river itself. In the United Kingdom, they may be known as overflow channels. Spillways ensure that water does not damage parts of the structure not designed to convey water.

San Gabriel River (California) River in Los Angeles County, California, United States

The San Gabriel River is a mostly urban waterway flowing 58 miles (93 km) southward through Los Angeles and Orange Counties, California in the United States. It is the central of three major rivers draining the Greater Los Angeles Area, the others being the Los Angeles River and Santa Ana River. The river's watershed stretches from the rugged San Gabriel Mountains to the heavily developed San Gabriel Valley and a significant part of the Los Angeles coastal plain, emptying into the Pacific Ocean between the cities of Long Beach and Seal Beach.

Detention basin Flood control measure

A detention basin or retarding basin is an excavated area installed on, or adjacent to, tributaries of rivers, streams, lakes or bays to protect against flooding and, in some cases, downstream erosion by storing water for a limited period of time. These basins are also called dry ponds, holding ponds or dry detention basins if no permanent pool of water exists.

Retention basin Artificial pond for stormwater runoff

A retention basin, sometimes called a wet pond,wet detention basin, or stormwater management pond (SWMP), is an artificial pond with vegetation around the perimeter, and includes a permanent pool of water in its design. It is used to manage stormwater runoff, for protection against flooding, for erosion control, and to improve water quality in an adjacent bodies of water.

The United States Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) is a dynamic rainfall–runoff–subsurface runoff simulation model used for single-event to long-term (continuous) simulation of the surface/subsurface hydrology quantity and quality from primarily urban/suburban areas. It can simulate the Rainfall- runoff, runoff, evaporation, infiltration and groundwater connection for roots, streets, grassed areas, rain gardens and ditches and pipes, for example. The hydrology component of SWMM operates on a collection of subcatchment areas divided into impervious and pervious areas with and without depression storage to predict runoff and pollutant loads from precipitation, evaporation and infiltration losses from each of the subcatchment. Besides, low impact development (LID) and best management practice areas on the subcatchment can be modeled to reduce the impervious and pervious runoff. The routing or hydraulics section of SWMM transports this water and possible associated water quality constituents through a system of closed pipes, open channels, storage/treatment devices, ponds, storages, pumps, orifices, weirs, outlets, outfalls and other regulators.

Cache Creek (Sacramento River tributary) Stream from Lake to Yolo Counties, CA

Cache Creek is an 87-mile-long (140 km) stream in Lake, Colusa and Yolo counties, California.

Los Angeles flood of 1938 1938 flood in Southern California, United States

The Los Angeles flood of 1938 was one of the largest floods in the history of Los Angeles, Orange, and Riverside Counties in southern California. The flood was caused by two Pacific storms that swept across the Los Angeles Basin in February-March 1938 and generated almost one year's worth of precipitation in just a few days. Between 113–115 people were killed by the flooding. The Los Angeles, San Gabriel, and Santa Ana Rivers burst their banks, inundating much of the coastal plain, the San Fernando and San Gabriel Valleys, and the Inland Empire. Flood control structures spared parts of Los Angeles County from destruction, while Orange and Riverside Counties experienced more damage.

San Juan Creek River in Orange County, California, United States

San Juan Creek, also called the San Juan River, is a 29-mile (47 km) long stream in Orange and Riverside Counties, draining a watershed of 133.9 square miles (347 km2). Its mainstem begins in the southern Santa Ana Mountains in the Cleveland National Forest. It winds west and south through San Juan Canyon, and is joined by Arroyo Trabuco as it passes through San Juan Capistrano. It flows into the Pacific Ocean at Doheny State Beach. San Juan Canyon provides a major part of the route for California State Route 74.

Urban runoff Surface runoff of rainwater created by urbanization

Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces are constructed during land development. During rain, storms and other precipitation events, these surfaces, along with rooftops, carry polluted stormwater to storm drains, instead of allowing the water to percolate through soil. This causes lowering of the water table and flooding since the amount of water that remains on the surface is greater. Most municipal storm sewer systems discharge stormwater, untreated, to streams, rivers and bays. This excess water can also make its way into people's properties through basement backups and seepage through building wall and floors.

River Natural flowing watercourse

A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of water. Small rivers can be referred to using names such as stream, creek, brook, rivulet, and rill. There are no official definitions for the generic term river as applied to geographic features, although in some countries or communities a stream is defined by its size. Many names for small rivers are specific to geographic location; examples are "run" in some parts of the United States, "burn" in Scotland and northeast England, and "beck" in northern England. Sometimes a river is defined as being larger than a creek, but not always: the language is vague.

Check dam Small dam to counteract erosion

A check dams is a small, sometimes temporary, dam constructed across a swale, drainage ditch, or waterway to counteract erosion by reducing water flow velocity. Check dams themselves are not a type of new technology; rather, they are an ancient technique dating from the second century A.D. Check dams are typically, though not always, implemented in a system of several dams situated at regular intervals across the area of interest.

Boeing Creek Stream in Shoreline, United States

Boeing Creek is a stream in the U.S. state of Washington, located in the city of Shoreline, just north of Seattle. It is about 1.6 miles (2.6 km) long and empties into Puget Sound. The creek is heavily modified along its course, and in many places has been diverted into culverts. The watershed of Boeing Creek is about 11.2 square miles (29 km2) in size, with two main tributaries aside from the mainstem. The creek takes its name from William Boeing, who built a mansion along the creek in 1913. Despite the river modifications and stormwater pollution, the creek supports a variety of riparian habitats, native animals and fishes.

Stony Creek (Sacramento River tributary)

Stony Creek is a 73.5-mile (118.3 km)-long seasonal river in Northern California. It is a tributary of the Sacramento River, draining a watershed of more than 700 square miles (1,800 km2) on the west side of the Sacramento Valley in Glenn, Colusa, Lake and Tehama Counties.

Chino Creek

Chino Creek is a major stream of the Pomona Valley, in the western Inland Empire region of Southern California. It is a tributary of the Santa Ana River.

Matilija Dam Arch dam in Ventura County, California

Matilija Dam is a concrete arch dam in Ventura County, California, completed in 1947. Designed for water storage and flood control, it impounds Matilija Creek to create the Matilija Reservoir in the Los Padres National Forest, south of the Matilija Wilderness and north of Ojai.

References

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